A wide variety of applications relating to chemistry involve the characterization and identification of the substances constituting multi-component blends, from laboratory research samples to large-scale reaction products which ultimately become commodity and specialty chemicals. To facilitate analysis, these blends generally are combined with a solvent component for dissolving components in the blend and diluting the blend to form a homogeneous multi-component mixture. One technique which is commonly used to separate these mixture components for analysis is gas chromatography.
In a gas chromatograph, components of a multi-component mixture are separated to aid in determining the chemical composition of the components and the quantity of each by sampling the components using a detector such as a flame ionization detector. Separation of the various components is effected by introducing the vaporized mixture by means of a carrier gas onto a column provided with either a solid stationary phase or a liquid stationary phase supported on an inert solid matrix. The sample components in the gaseous phase differ in their tendency to either adsorb onto the solid stationary phase, or to dissolve in the stationary liquid phase. When the carrier gas and vaporized sample are forced through the column, the rate of movement of the various components of the sample depends upon their tendency to either adsorb onto that solid phase or dissolve in that liquid phase. If the component is readily adsorbable or soluble in the particular solid or liquid stationary phase, the component will elute or pass slowly from that phase. On the other hand, if the component is only marginally adsorbable or soluble in the particular stationary phase, it will move rapidly through the column.
As the blend passes through the column, and as a result of the differing adsorption or solubility rates, the vaporized mixture of components eventually separates into a plurality of serially discrete gaseous "plugs" of pure single components; or, in other words, into a train of plugs, each having a homogeneous composition of one of the components to be analyzed. A detector, such as a flame ionization detector, registers the time elapsed from injection of the mixture to the passage of the respective components through the column by causing a deflection in the shape of a bell curve on a moving chart as a plug of pure gaseous component passes over the detector. Thus, a qualitative rendering corresponding to the elution of the component is obtained.
It is desirable, further, that the amount or quantity of the component of the mixture be determined. This is typically obtained by measuring the area under the bell curve.
Action of the carrier gas alone does not cause efficient elution of all the mixture components from the stationary phase. The typical means for facilitating elution is to also adjust the temperature of the column as the run progresses, beginning at a base temperature and increasing linearly over time to the final temperature to aid in removing the components from the stationary phase so that the separation can be completed in a reasonable time. The base temperature, final temperature and rate of increase are all separately adjustable prior to the run. At the final temperature, all the components should be removed from the stationary phase. Typically, the flow rate of the carrier gas during the run remains constant.
There are certain problems associated with temperature programmable chromatography, where the temperature, for example, is increased at a linear rate, or "ramped" up from a base temperature. First, easily decomposable components may be destroyed within the column as the temperature increases. Also, once the unit has reached the final temperature, any subsequent run cannot be conducted until the column has cooled to the base temperature. Further, the evaporation rate of the stationary liquid phase in the column changes exponentially with temperature, thus leading to accelerated degradation of the column at high temperatures.
As an alternative method, separation may be accomplished by adjusting the pressure of the carrier gas as the run progresses while maintaining a generally constant temperature. There are certain advantages to this type of separation method. First, compounds having high boiling points are able to be eluted at relatively low column temperatures. Also, the column lifetime is extended relative to a column subjected to temperature ramping. Further, the evaporation rate of the stationary liquid phase changes only linearly with flow rate compared with an exponential change in a temperature ramped device, the flow rate varying approximately linearly with the changes in pressure encountered in the gas chromatograph. Easily decomposable compounds can be analyzed with less risk of destruction. Finally, the turnaround time is improved because the pressure can be cycled back to the base value more quickly than temperature, thus allowing more efficient use of instrument time.
As with temperature-adjustable gas chromatography, the pressure can be linearly varied with time, or "ramped". One type of pressure ramped gas chromatography involves use of a procedure known as split injection. Depending on the diameter of the column, the gas chromatograph may process the entire volume of sample mixture which is injected, or alternatively may require only a portion of that sample mixture and vent the remainder. The latter technique is known as split injection. Split injection is particularly useful in gas chromatographs utilizing low cross-sectional area columns having inside diameters in the range of 50 to 320 microns (ID), known as capillary columns. The means to ramp the pressure will be described below.
Where it is desired to determine the actual quantity of each component in the sample mixture using the split injection technique, one must maintain a constant ratio for the amount of sample mixture in the column to that amount which is vented. One problem encountered in split injection pressure ramped chromatography is the inability to obtain data to precisely quantitate the amounts of each component separated within the column. It is imperative for quantitation that the pressure within the column at the time of sample mixture injection varies solely in response to the contribution of the sample mixture injected onto the column. Therefore, the pressure regulator unit for the gas chromatograph is generally downstream of the sample injection point, thereby acting as a back pressure regulator. The goal in split injection pressure-ramped gas chromatography is to maintain a constant flow rate during vaporization. To adjust the pressure within the column, it is known to use an adjustable pressure regulator having a needle valve. Though controlled flow is obtained with this type of regulator, one eventually experiences the problem that the needle valve tends to become plugged after a number of successive runs, resulting in varying rates of flow which cause unreliable results. The plugged valve must then be cleaned or replaced.
It is possible to utilize a back pressure regulator having a flat seat and valve plunger wherein the pressure is adjusted by oscillating the valve plunger at varying frequencies to vary the amount of contact with the valve seat. Thus, the plunger is maintained apart from the valve seat for a longer period of time at the low pressure end of the ramp, and maintained in contact with the seat for longer periods at the high pressure end of the ramp. The regulator's pressure response time is improved by controlling the oscillation electronically. This regulator is known as an oscillating electronic back pressure regulator (OEBPR).
As noted above, an OEBPR regulates pressure by adjusting the oscillation rate of a plunger making contact with a valve seat. In the case where the pressure within the tubing rises rapidly due to vaporization of the mixture at the injector, the OEBPR attempts to compensate for the build-up by adjusting the oscillation frequency. However, the pressure increase due to vaporization and subsequent adjustment by the OEBPR results in pressure fluctuation in the gas chromatograph in the vicinity of the injector outlet and the vent. In split injection chromatography, this fluctuation ultimately results in a loss of precision in quantifying the component amounts present as they travel through the column and pass over the detector. In other words, the vagaries of the response time of the OEBPR may result in an indeterminate amount of sample loss, adversely affecting chromatograph sample vaporization accuracy, i.e., OEBPR response time is too slow to avoid significant unknown sample loss. In addition, due to the oscillating nature of the OEBPR, pressure pulses are observable in the injector area which translate to small changes in the actual split ratio in the injector during sample injection. The split ratio is defined as the ratio of the actual flow diverted to a vent divided by the actual flow in the column. The small changes in the split ratio are due to pressure pulses in the injector which reduce the reproducibility, or precision, of repetitive injections of a given sample.
It is therefore an object of the invention to provide an improved pressure regulator apparatus for a pressure programmable gas chromatograph having reduced risk of plugging, but without indefinite undue sample loss.
It is yet a further object of the invention to provide an improved pressure regulator apparatus for a pressure programmable gas chromatograph where undesirable pressure fluctuations are substantially eliminated.
It is yet a further object of the invention to provide an improved pressure programmable gas chromatograph with improved component quantitation and reduced tendency of regulator valve plugging.
It is yet a further object of the invention to provide an improved method for effecting pressure programmable gas chromatography.